Murph, I assume that you meant DPLIIx, since DPLII doesn't work on multi-channel material. If so, the general view is that having speakers in back to image sounds which are intended to come from behind is an advantage.
Since DPLIIx does in fact extract material from the side surrounds in 5.1 material(which is about equal in level in both surrounds and is supposed to represent sounds from the back), they don't play that material, and if the back surrounds are far poorer, as you suggest, the result of using them might be disappointing.
